Objectives To develop reagents for the improved diagnostic serology of Lyme disease. Results An antigen of the spirochete Borrelia burgdorferi (rP7-1) was identified, cloned and used as a probe for the early serological diagnosis of Lyme disease. Serum samples obtained from rhesus monkeys infected with three different strains of B. burgdorferi, B31, JD1 and NT1, contained a significant concentration of antibody to purified rP7-1 by week 2 of infection, as detected by ELISA. Both IgM and IgG antibodies were tested simultaneously. The result indicates that antibody to rP7-1 appears early in the course of infection, regardless of the strain of B. burgdorferi that is eliciting the antibody response. Sensitivity of antibody detection was assessed by Western blot with a battery of 43 human serum samples obtained from the CDC. The experiment was performed in a blinded fashion. The CDC samples had been obtained from patients with a clinical diagnosis of Lyme disease that satisfied the stringent CDC criteria for case definition. Of the 43 samples from clinically positive patients, 34 were reported positive by the CDC, using a commercially available Western blot assay based on detection of multiple bands simultaneously. With the diagnostic Western blot based on the rP7-1 antigen, an equal number of samples were positive (34). Thus, the rP7-1-based assay, in which a single band is (or is not) detected and which is therefore a test simple to interpret, yielded the same sensitivity as one of the most accomplished -yet cumbersome to interpret- Lyme disease diagnostic tests presently in the market. We are currently testing specificity. So far, from 12 serum samples of syphilitic patients, 11 were negative with our assay despite the fact that all of the samples contained antibodies that cross-reacted with multiple antigens on a Western blot of whole B. burgdorferi antigens. The single serum sample that gave a positive result was from a patient who also had and HIV infection and several AIDS-related infections. Future directions To extend our survey of crossreactivity; samples from patients with autoimmune diseases will be tested.